This invention relates to a pattern-matching sewing machine, for sewing two sheets, such as cloths, each bearing the same patterns with the patterns matching.
Published Unexamined Japanese Patent Application No. S60-153896 (which corresponds to the U.S. Pat. No. 4,612,867, and the German Patent Application DE 33 46 163 C1) discloses a pattern-matching sewing machine of this type. In this machine, a photo-sensor is placed before the sewing point to generate intensity data representing the brightness of the patterns on the two cloths. The mismatch distance of the patterns on the two cloths is detected using the intensity data, and the relative feed amount of the two cloths is adjusted according to the mismatch distance to maintain the pattern match.
However, the sewing machine, after collecting a predetermined number of photo-intensity data and adjusting the mismatch distance by relatively moving one cloth to the other (at q in FIG. 16), waits till the predetermined number of photo-intensity data are collected (r) again. Then, the next mismatch distance is calculated (s) and adjusted (t). A step motor for adjusting the mismatch distance is driven only at the adjustment of the mismatch distance (q and t), so its dormant period is long (530 msec). Especially, when the pattern recurring is long, i.e., when the predetermined number is large, the step motor adjusts the mismatch distance fewer times, resulting in slow response and inaccuracy in the pattern-matching control. Further, when the step motor adjusts a long mismatch distance at a time, either of the two cloths may wrinkle. That is, one adjustment of the mismatch distance should be small.